In 2004 the mood was “Stay the Course”. GW Won re-election
In 2000 the mood was “It’s time for a change” after 8 years of Clinton and his percieved flaws in moral character. The Democrats lost, allowing GW to take office.
In 1996 the economy was soaring, and the mood was “Stay the Course”. Clinton easily won re-election.
In 1992 the economy was terrible, people wanted change, and Bush Sr. lost to Clinton.
etc, etc.
Additionally, history tells us that the more charismatic candidate almost always wins in Presidential Elections. GW is more charismatic than either Kerry or Gore. Clinton was more likable than Dole or Bush Sr.
Given the current economic climate and drawn out military action, it seems like people pretty strongly want change. Obama is more charismatic than McCain.